1. Field of the Invention
The present invention relates to a spacer-type composite overhead ground wire in which stranded metal wire is wound on an optical unit comprising a grooved aluminum spacer containing an optical fiber. More particularly, the invention relates to an optical unit for a composite overhead ground wire having an improved covering for holding down the optical fiber contained in the optical unit.
2. Background Art
FIG. 1 is a schematic structural drawing of a conventional optical unit for a spacer-type composite overhead ground wire. The optical unit contains an optical fiber 3 in each of the grooves of a grooved aluminum spacer 2. In order to prevent the optical fiber 3 contained in the groove of the spacer 2 from protruding therefrom, the process of making such an optical unit for a composite overhead ground wire includes fitting and packing the optical fiber 3 into the groove of spacer 2, winding an aluminum foil 5 less than 40 micrometers thick on the outer periphery of the grooved aluminum spacer 2 at a pitch of roughly 1/10, and encasing the spacer in an aluminum pipe 4.
The covering 5 wound on the optical unit for the composite overhead ground wire thus constructed must meet the following characteristics and requirements:
(1) electrical conduction between the grooved aluminum spacer 2 and the aluminum pipe 4;
(2) instantaneous heat resistance in the range of about 300.degree. C.;
(3) thickness of about 40 micrometers or less to make the covering layer as thin as possible; and
(4) tear-free properties during manufacture.
The aluminum foil conventionally used as the covering of an optical unit for a composite overhead ground wire of that type has properties characteristic of a weak foil. In particular, the presence of a scratch at an edge will readily develop into a tear. The problem is that during production the grooved spacer rubs against a guiding conduit or two grooved spacers rub against each other. This rubbing readily results in the breakdown of the edges of the aluminum foil. If the aluminum foil used as a covering is slightly torn, the aluminum pipe will be clogged with it in the next processing step in which the aluminum spacer is encased in the aluminum pipe. Thereby, defective cable will be produced.